When drilling a subsea subterranean borehole for oil and/or gas production, it is known to use a tubular drill string which extends down from a drilling rig at the ocean surface into the borehole through a wellhead mounted at the ocean floor. The drill string has a drill bit mounted at its lowermost end and drilling may be achieved by rotating the drill string using a top drive mounted on the drilling rig, or by rotating the drill bit using a downhole motor at the remote end of the drill string. A tubular riser is mounted on a blowout preventer (BOP) provided at the top of the wellhead, and extends generally vertically upwardly to the ocean surface, whilst the drill string extends down the riser into the borehole.
During drilling, a fluid (known as drilling mud) is pumped down the inside of the tubular drill string, through the drill bit, and circulated continuously back to surface via the drilled space between the borehole and the drill string (referred to as the wellbore annulus), and between the riser and the drill string (referred to as the riser annulus). The riser thus provides a flow conduit for the drilling fluid and cuttings returns to be returned to the surface to the rig's fluid treatment system.
Deepwater drilling risers were traditionally designed as a conduit for transporting well bore returns to the rig during conventional drilling operations or for diverting returns overboard during conventional well control in the event of a shallow gas kick or an influx escaping past the subsea BOP. In such systems, the riser is designed as a flow conduit that is open to atmospheric pressure and is not a pressure containment system.
Since the development of riser flow control drilling systems, a drilling operation is now able to apply a safe amount of back pressure to the riser for the purposes of managed pressure drilling or reducing peak gas flow rates in a riser gas event. A riser flow control system consists of a pressure control manifold on the rig and a riser sealing device that diverts returns to the pressure control manifold. Where the riser is used in this way, there is a need to include a continuously available pressure relief system which provides an alternative flow path out of the riser for drilling returns so that the weakest link in the riser system is not over-pressured in the event of a control system failure, an operational error or a blockage in the conduit normally transporting riser returns to the rig.
Electrically operated pressure relief systems which use a PLC and pressure transducer to signal the actuator of the pressure relief valve have previously been described, and are disclosed, for example, in U.S. Pat. No. 4,636,934 and US 2011/0098946. In the event of an umbilical failure, or a failure of the electronic control system, the electrical communication required to operate such a system may be lost, and this can cause the system to be unavailable when needed or result in an unintended actuation (opening) of the pressure relief valve. An unintended actuation can cause an environmental hazard by diverting oil based drilling mud overboard unnecessarily (because there was no over-pressure event to begin with). Alternatively, a lack of system availability during a riser over-pressure event can cause the riser to burst through resulting in danger to the rig crew as well as an environmental hazard. To avoid this, the system must be provided with full redundancy, which involves providing multiple umbilicals, PLCs, pressure transducers, etc. at significant cost.